This invention relates to methods and apparatus for transmitting and receiving frames on a local area network (LAN).
This invention relates particularly to methods and apparatus for controlling the receiving and transmitting of data frames into and from a bridge of the kind which interconnects two or more LANs and which operates in two different modes. In one mode the bridge retransmits a frame with a previously appended check sum. In the other mode the bridge responds to a query for information by generating a frame from an associated memory and by then computing and appending a frame check sum to that frame.
The present invention relates particularly to methods and apparatus for controlling the receiving and transmitting of data frames into and from a bridge of this kind. The present invention insures that the bridge receives all frames transmitted to it without the risk of loss of frames during any period of time in which the controller is being reprogrammed for a switch between the two different modes of operation.
This invention relates to a controller for a bridge of the kind disclosed in co-pending U.S. patent application Ser. No. 06/682,061 filed Dec. 14, 1984 entitled "Method and Apparatus for Bridging Local Area Networks" and assigned to the same assignee as the assignee of this invention. This co-pending U.S. patent application Ser. No. 06/682,061 is incorporated by reference in has been issued as U.S. Pat. No. 4,706,081 on Nov. 10, 1987, and this application in accordance with the provisions of the Manual of Patent Examining Procedure, Section 608.01(p)B.
Ethernet and IEEE 802.3 LANs are being installed in conjunction with a wide variety of office automation and data communication products. The LANs are used to locally interconnect a number of products built by numerous vendors (e.g. Digital Equipment Corporation, Intel, Ungerman/Bass, 3COM, etc.).
The LAN interface for LAN products is relatively simple. It always computes and appends a frame check sum (FCS) to transmitted frames and validates and removes the FCS from received frames. It cannot switch to transmitting a frame which already contains an FCS without the risk of not receiving frames simultaneously being transmitted by other stations on the LAN. Also, this interface is only required to receive LAN frames addressed to the product (not all LAN frames).
Devices which interconnect remote LANs and all the vendors' products attached to the remove LANs are termed data link bridges. Bridges have the unique LAN interface requirements addressed by this invention.
The invention relates particularly to transmitting frames on an Ethernet or IEEE 802.3 LAN. In particular, some transmitted frames already have an appended FCS value. In this case the interface must not compute and append a second FCS value. When the transmitted frame does not have an FCS value the interface must compute and append the FCS value.
The invention also relates particularly to receiving all frames transmitted by other stations on the LAN. The interface first computes and verifies that a received frame and its FCS value agree and then, if they do, the frame with its valid FCS is passed to the bridge.
It is an important objective of the present invention to attach data link bridges to Ethernet and 802.3 LANS.
It is a specific objective of the present invention to receive all frames transmitted on the LAN by other stations, back to back, or otherwise, while transmitting frames, some of which already contain the appended FCS value.
It is a related objective to compute and verify that each frame received from the LAN has a valid FCS value.
It is a related objective to discard each frame received from the LAN with an invalid FCS and to pass to the bridge all other of the received frames with their valid FCS values still appended.
It is a related objective to be able to switch between transmitting a frame already containing an FCS to transmitting a frame which must have the FCS computed and appended while continuing to receive all frames being transmitted by other stations on the LAN.
In the present invention there is a basic principle of operation involved in the operation of the interface.
This basic principle of operation is the ability to transmit both frames which already contain an FCS and frames which must have the FCS computed and appended while continuing to receive all frames being transmitted by the other stations on the LAN.